Hotel cancelled booking?

Practically speaking, you'll probably need to rebook your accommodations (whether at that hotel or at another). You could pursue the matter in court against the hotel in question, but it will probably take quite a while to resolve, and the odds of success aren't necessarily high. It would come down to whether the price was a genuine error (as opposed to some deliberate bait-and-switch tactic), how much lower the advertised price was, and how long it took the hotel to notice and correct the error. If the incorrect price was so low that a reasonable customer should have good reason to believe it was a mistake, or if the hotel cancelled your booking right away and immediately corrected the price on their site so that it is no longer being advertised for the erroneous rate, that would tend to tilt things in their favour and would make it less likely that you'd be successful in pursuing the matter.

Before rebooking, you may want to ring the hotel and discuss things with a manager; just be polite, but explain the situation and how the hotel's mistake has made things difficult for you, and see if there's anything they can do to help you. While you probably won't get the off-season rate, you might be able to get some sort of discount or free upgrade or something from them for your trouble. If that doesn't work, you could try contacting them via social media (Twitter, Facebook, etc.) and stating your case; that sometimes gets better results, as companies don't like the negative publicity that comes from such posts if they go unanswered or unresolved.